Referring to FIG. 1, WO2010/072710 discloses a power plant having a gas turbine unit 1 that comprises a compressor 2, a combustion chamber 3 and a turbine 4.
A mixture 6 comprising fresh air 7 coming from the environment is fed into the compressor 2 and flue gases 8 (deriving from the combustion of the mixture 6 with a fuel within the combustion chamber 3) emerge from the turbine 4.
These flue gases 8 (that typically have a high temperature) are preferably fed into a boiler 9 of a steam turbine unit 10; within the boiler 9 the flue gases 8 transfer heat to water of the steam unit 10.
From the boiler 9, the flue gases 8 are supplied into a diverter 11, to be split into a recirculated flow 12 and a discharged flow 13.
The recirculated flow 12 is cooled in a cooler 14 provided on a flow path for the same recirculated flow 12; then the recirculated flow 12 is supplied via a fan 15 into a mixer 16, to be mixed with the fresh air 7 and form the mixture 6 that is fed into the compressor 2.
The discharged flow 13 is cooled in a cooler 19 and is then fed, via a fan 20, into a CO2 capture unit 21 to be then discharged into the atmosphere via 22; in contrast the CO2 that is captured in the CO2 capture unit 21 is stored in 24.
Because of the flue gas recirculation, the amount of oxygen at the compressor inlet is lower than its amount in the fresh air. In this respect, the fresh air oxygen concentration is typically around 21 mol %, whereas its concentration at the compressor inlet of a power plant with flue gas recirculation is lower or much lower than 21 mol %.
It is clear that in case the oxygen amount at the compressor inlet is too low, within the combustion chamber 3 the oxygen amount would also be too low and could also fall below the stoichiometric amount (i.e. the minimum amount theoretically needed to achieve complete combustion). In this case incomplete combustion, with high CO, unburned hydro carbons and eventually flame extinction, could occur.
In case a plurality of combustion chambers, with a downstream combustion chamber fed with the flue gases still rich in oxygen coming from an upstream combustion chamber is used (sequential combustion gas turbine unit), this problem is even more severe.
For these reasons, measurement of the oxygen concentration at the gas turbine unit compressor inlet is required.
Nevertheless, the flow conditions upstream of the compressor inlet are very complex and are characterised by high vortices, turbulence and ongoing mixing. In practice, a direct oxygen concentration measure can not be carried out.